Bowser

Bowser is the main villain of the Mario game series. At the conclusion of each battle, Bowser sometimes survives the beating or whatever happens afterwards, but on some occasions he appears to actually be killed, though in each case he always returns to try and kidnap Peach and beat Mario again. In New Super Mario Bros. he is revived onscreen by his son, Bowser Jr. This is lampshaded in Paper Mario: The Thousand-Year Door, when the player plays as Bowser after Chapters 2, 4, and 6, in Super Mario Bros. style levels, he has infinite lives.

It is unclear what chronological order all the Mario games take place in, or whether they all even take place in the same universe. Thus, which is Bowser's final, single unrevived death, or whether he gets one at all, is a mystery.

Mario and Luigi chopped down the bridge with an axe, sending him falling into the lava. (Super Mario Bros.)

The intensity of the battle between Bowser and Mario caused his machine to explode and send him flying into the distance. (Paper Mario, confirmed in Luigi's Mansion but returns in Super Mario Sunshine)

Mario and Luigi hit a skull switch causing the bridge to collapse and send Bowser falling into lava again, reducing him to bones. Was later revived onscreen by Bowser Jr. (New Super Mario Bros.)

Castle fell on top of him and his eight children, crushing them. If New Super Mario Bros. Wii is not the last game chronologically, then there is some question as to how he returned in later games, though it is conceivable that Magikoopas brought him back.

Trapped in an energy box and blown up by Dimentio, along with Mario and Peach. This wasn't intended to permanently kill him, only temporarily, so that he and the others were still very much alive in the afterlife (sometimes called the aftergame), and were able to return to other dimensions.

Fell into the lava due to Mario hitting a switch in Super Mario 3D Land. Was later revived at some point in the game.

Fell into the lava again in Super Mario 3D Land, the same way as last time.

Fell into the lava in Super Mario 3D World, when Mario, Luigi, Peach, and Blue Toad caused his car to smash against the bridge which crumbled beneath him. He was revived immediately after.

Mario, Luigi, Peach, and Blue Toad struck a giant POW Block he was standing on, sending him flying into the distance where he explodes in a Bowser-shaped fireworks explosion.

There is also some question as to whether he fell to his death in Super Mario Bros. 3, but as he is shown to survive a similar fall in New Super Mario Bros., with Bowser Jr. dragging him away by the tail and scowling at the audience. It is also unknown whether at the end of Super Mario 64 he was destroyed or merely teleported away, however he did say "Come on, troops! Let's watch the ending together!" Indicating that he survived.

He was ALMOST crushed to death on several occasions, most of them in Mario & Luigi: Bowser's Inside Story, in which Mario and Luigi ford the adrenaline channel to make him Giant Bowser. He was also seemingly crushed to death along with his rival O'Chunks when they held the falling ceiling up to save Mario, Luigi, and Peach, however, as what often seems to happen with Bowser lately, his weight caused him (and O'Chunks as well) to fall through the floor and survive.